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What this video covers

A diesel engine reading a steady 165°F (75°C) under load sits at the low end of the normal operating range but is not automatically a problem.1:14 Nigel Calder explains that engines are designed to run at widely different temperatures — his own Volvo Penta D275, for example, runs at 93°C, close to boiling.0:48 The first step is to check the engine manual for the manufacturer's specified operating temperature.1:01 A consistent reading at any temperature suggests the thermostat is functioning; the system is working, just possibly calibrated wrong.1:34 If the temperature is lower than the manual specifies, the likely cause is a thermostat set to the wrong opening temperature rather than a failed one — a stuck-open thermostat would typically produce erratic readings, not a steady low one.1:34 On older engines with unknown service histories, a previous owner may have installed an incorrect thermostat.Watch Calder distinguishes between a broken thermostat and a wrong-spec thermostat, noting the latter is the more probable culprit when temperature is consistently low but stable.1:34

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